To provide a disposable diaper enabling reduction in re-wet amount and having an excellent speed of incorporating liquid regardless of concentration and configuration of a water-absorbing agent in an absorbent material. A water-absorbing agent having excellent Gel Capillary Absorption (GCA) and Free Gel Bed Permeability (FGBP) is obtained by crushing a crosslinked hydrogel polymer obtained in a polymerization step to have a specific weight average particle diameter while fluid retention capacity and a surface tension of a water-absorbing agent are adjusted in a specific range, drying the crushed crosslinked hydrogel polymer, and then adding a liquid permeability enhancer thereto during surface crosslinking or after surface crosslinking.
